[v13] Omit empty fields from DeviceCredential resources

[codingllama]

Backport #27821 to branch/v13

Omits, from the resource representation of devices, fields that are always empty
in some cases.

DeviceAttestationType is particular in the sense that it tends to show up as
"unspecified", while other enums are never unspecified in valid devices (OSType,
EnrollStatus), or tend to have their entire parent message/struct nil when they
would be unspecified (DeviceOrigin).

-before/+after:

tctl get device/foo
kind: device
metadata:
  name: 9c39701e-9f19-4766-bc0c-1241d08ce649
spec:
  asset_tag: foo
  create_time: "2023-06-13T17:47:51.169251Z"
  credential:
-   device_attestation_type: unspecified
    id: ae2d978c-fee8-419d-a2e6-a5dd0a00c4b8
    public_key_der: MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEg1iznap59moe9gDCySvK1dYkenZBgRdif1No8u4sjlhp7qyRjMxSoFv8wytBCZNSJRRZ55prKVlMEjxcdFtJlQ==
-   tpm_ak_public: null
-   tpm_ekcert_serial: ""
  # so on ...
version: v1

https://github.com/gravitational/teleport.e/issues/827